Jungner battery, that is nickel cadmium batteries (Ni/Cd batteries), invented in the early 20th century, enabled to be sealed in 1940s. Ni/Cd batteries have been developed remarkably as secondary batteries which represent alkaline storage batteries with the expansion of the use of small-sized secondary power source, including in such fields as home electric, communications, office tools, sundries use, or the like, since around the year 1970. However, around the year 1990, having a hydrogen absorbing alloy electrode instead of having a cadmium electrode is developed newly, and these nickel-metal hydride storage batteries have been exceeding the Ni/Cd batteries in production and sales amount as a new power source for portable electronic devices with lithium secondary batteries in recent years.
In addition to the above mentioned use, since the market for hybrid electric vehicles (HEVS) put on the market in 1997, electric automobiles, electric scooters, and electric-assisted bicycles is anticipated to expand, the development of highly efficient secondary batteries as the power source thereof (hereafter called power source for mobile use) is expected.
The use as the said power source for mobile use requires high power among the characteristics which are necessary for general use. Further, it is also required to have high reliability and high energy density (small sized and light weighted).
Since alkaline storage batteries are capable of showing these characteristics relatively strongly, they are drawing attention as candidates of power source for mobile use. In particular, Ni/MH batteries among alkaline storage batteries have already been equipped with mass produced HEVs and have been drawing the greatest attention with the reason that they have the image of using clean materials and have high energy density.
Therefore, for the purpose of explaining specifically, as battery system for high power use, which is the aim of the present invention, the explanation follows taking Ni/MH batteries, in particular, sealed cylindrical Ni/MH batteries as an example.
Ni/MH batteries, like Ni/Cd batteries, belong to alkaline storage batteries of 1.2 V systems and have relatively high energy density and high reliability. However, Ni/MH batteries are slightly inferior to Ni/Cd batteries in high power. Therefore, Ni/MH batteries have some problems to decrease the feature of high energy density which is the characteristic inherent to these batteries as a result of volume increase of both positive and negative electrodes' substrate and separators when the batteries are so constructed as to show high power by employing thin and long positive and negative electrodes. In other words, at present, high energy density Ni/MH batteries do not have satisfactory high power characteristics. Therefore, the improvement of high power characteristics is strongly desired.
Heretofore, in producing high powered alkaline storage batteries, many efforts have been made to lower the internal impedance of the batteries including the selection of materials for positive and negative electrodes, materials for electrolyte and the concentration thereof, and further, the improvement in the construction method of both electrodes and batteries. However, the improvement of the separators is drawing attention which deeply relates to the progress in the rate of ionic velocity in passing through the separator between the positive and negative electrodes, or high-powered batteries, since, in some cases, it is even more effective than the above mentioned items, and also plays a great role in stabilizing the self discharge at a low level as a secondary effect. Therefore, further improvement of the separators is desired for high powered Ni/MH batteries.
In addition, there are four essential requirements of the separator for sealed Ni/MH batteries as described below which are common to the general sealed alkaline batteries.
(1) The materials are chemically stable to the electrolyte, or the like.
(2) The negative electrode and the shedding thereof are separated from the positive electrode and the shedding thereof in order to prevent short circuit.
(3) The appropriate amount of the electrolyte is contained in the pores inside of the separator.
(4) There are some appropriate pores so that the oxygen gas generated in the positive electrode can go through.
For the development of the separators used for the batteries with high power use, it is regarded as important that the separators should have the characteristics of low impedance and high reliability, while meeting the above described requirements. In the case that the separators with high reliability of preventing short circuit can be developed, by making separators much more thinner, the concerned extreme decrease of the energy density can be prevented even though the positive and negative electrodes are processed to be thin and long for high power use.
In the past, the non-woven separators which comprise polyamide type resin fibers as materials for separators in mass produced Ni/Cd batteries were used, mainly in sealed cylindrical Ni/Cd batteries, and the separators using this resin fiber are currently used.
On the other hand, as for sealed cylindrical Ni/MH batteries, the problem arises due to the extremely large self discharge in the case of using the conventional polyamide type separators. However, this problem is no more a fatal defect as reported in the 171 th ECS (USA) Fall Mtg, Ext. Abst., Vol. 88–2127(1988), J, Electrochem. Soc., Vol. 143, No6, 1904(1996), or the like by employing polyolefin type separators which are chemically stable to the alkaline electrolytes. It seems that this problem is due to the sooner decomposition of the positive electrode active materials caused by ammonium, nitrous acid, or nitrate ion obtained slightly as the decomposition products of polyamide type resin in the sealed Ni/MH batteries in which hydrogen gas always exists different from the sealed Ni/Cd batteries' case.
However, the polyolefin type separator is generally not easy to get wet with aqueous solution, in other words, is hydrophobic. Therefore, polyolefin type separator requires treatment for providing hydrophilic property. Therefore, the following hydrophilic treatments have been already adopted industrially.    1) To treat with surfactant.    2) Method to graft the hydrophilic groups onto polyolefin with acrylic acid or the like.    3) Method to make sulfo group having hydrophilic property or other groups having the same property react on the surface of the said fiber chemically.
For the purpose of requiring high power, that is, for the purpose of realizing high rate discharge under the wide range of temperatures' atmosphere, it is important to maintain the hydrophilic property for a long time and the self discharge characteristics. From this viewpoint, the method 3) is the most excellent in providing hydrophilic property stably.
At present, it is general to adopt a method of sulfonation by immersing a non-woven separator in concentrated sulfuric acid at a high temperature. The said non-woven separator is prepared by entangling the said polyolefin type resin fiber, in particular, the core-sheath type polyolefin type resin fiber whose core consists of polypropylene type resin and whose surface consists of polyethylene type resin. These methods have been proposed in the unexamined Japanese Patent publication No. 01-132044 or in U.S. Pat. No. 5,100,723.
These proposals have already been employed in the batteries for consumers, or the like. For the high powered batteries, it is necessary to enhance the degree of sulfonation due to the decrease of the impedance within the batteries with the increase of the liquid-retaining property depending on the increase of the hydrophilic property of the separators as described above.
However, the degree of sulfonation, which is defined as the number of sulfur atoms (S) to the number of carbon atoms (C) in polyolefin resin, is not more than 3×10−3 to 5×10−3 with the method of immersing the non-woven cloth which comprises the usually used polyolefin type resin fiber having the diameter of about 10 μm into concentrated sulfuric acid at a high temperature or fuming sulfuric acid. This is because the necessary strength cannot be obtained in constructing spirally wound plates of the battery. This result is brought about by the extreme decrease in physical strength of the fiber itself. That is, when the said method is used, a part of a resin is sulfonated internally and sometimes even goes further to carbonization around the degree of little over 3×10−3. Thus, it has been difficult to expect further improvement in liquid retention property with the use of the conventional method.
This tendency has been more remarkable when the fiber diameter becomes narrower, and the introduction of non-woven cloth which comprises fine-spun fiber and by which the thinner separator is expected for its high reliability as a separator, becomes even more difficult. Therefore, when the electrode was made thin and long for high power use, it brought about the decrease in the energy density of the batteries since it becomes necessary that the separator with the conventional thickness should also be elongated.
However, it was found that the sulfonation method which is the reaction with SO3 gas can inhibit the carbonization inside the fiber further compared with the conventional method. It was also found that the physical strength of the fiber is hard to be reduced. In the above described patents, in addition to the method of using concentrated sulfuric acid at a high temperature or fumed sulfuric acid, the method to react with SO3 is also proposed although there is no detailed description specifically therein. However, reaction of non-woven cloth comprising polyolefin type fiber with only SO3 gas not merely has made the sulfonation extremely uneven but also hydrophilic property of the whole non-woven cloth lower, and the problem which is the increase of impedance arises. For information, the inner impedance in the conventional AA sized battery structure using the conventional sulfonated separator is 8 to 10 mΩ but the results have been obtained that the inner impedance in the battery structure substituting only the said conventional sulfonated separator with the separator merely reacting with SO3 gas with other conditions being equal increased to 10 to 13 mΩ.
Non-woven cloth comprising sulfonated polyolefin type resin fiber is an extremely important material for high power use or as a separator of Ni/MH batteries for high rate discharge at a high temperature. However, the conventional method has its limit of the increase in the degree of sulfonation, said increase is performed by providing further hydrophilic property necessary for high power use. In other words, the conventional method of processing with concentrated sulfuric acid or fumed sulfuric acid has the problem to be solved of causing the decrease in physical strength of the fiber, leading separators to break in constructing spirally wound electrodes. Therefore, sulfonation by SO3 gas which can inhibit the decrease in strength draws attention. However, just to react with SO3 gas only makes partial progress of sulfonation on the fiber surface and the uniform sulfonation as a whole is hard to be achieved. Further, the problem that the impedance within the battery increases on the contrary arises.